This invention relates to an apparatus for detecting a focus condition of an imaging lens provided in optical devices such as still camera, cine camera and microscope.
FIG. 1 is a schematic view illustrating a principal construction of a known focus detection apparatus using a focus detection principle of lateral image shift. The known apparatus comprises means for forming laterally shifted images consisting of a plurality of small lens aperture dividing optical systems 2.sub.k (1.ltoreq.k.ltoreq.n) for dividing an exit pupil of an imaging lens 1, and a light receiving element array 3 consisting of n pairs of light receiving elements 3.sub.k -A and 3.sub.k -B (1.ltoreq.k.ltoreq.n), each pair being arranged in relation to a respective one of the lens aperture dividing optical systems 2.sub.k so as to receive light fluxes emanating from different regions of the exit pupil of the imaging lens 1. The light receiving element array 3 is arranged in a predetermined focal plane of the imaging lens 1 or a plane conjugated with the focal plane or near these planes. By this construction, an odd numbered light receiving element array, i.e. a first element array 3.sub.1 -A, 3.sub.2 -A . . . 3.sub.n -A and an even numbered light receiving element array, i.e. a second element array 3.sub.1 -B, 3.sub.2 -B . . . 3.sub.n -B receive images which are laterally shifted with respect to each other in opposite directions in accordance with a defocus condition. Therefore, by suitably processing photoelectrically converted outputs from the first and second light receiving element arrays, it is possible to evaluate the focal condition of the imaging lens 1.
In the above known apparatus, the lens aperture dividing optical system comprises a stripe mask array 2 and the paired light receiving elements 3.sub.n -A and 3.sub.n -B of the array 3 are arranged symmetrically with respect to respective light transmitting portions 2.sub.n of the stripe mask array 2, so that the paired light receiving elements 3.sub.n -A and 3.sub.n -B receive separately the light fluxes emanating from the different regions of the exit pupil of imaging lens 1. Therefore, in order to detect the focal condition of the imaging lens 1 in an accurate manner, it is desired that the light receiving element pairs 3.sub.n -A and 3.sub.n -B receive the light fluxes which equally divide the exit pupil of the imaging lens 1.
However, as illustrated in FIG. 2, in the known focus detection apparatus, although paired light receiving elements 3.sub.l -A and 3.sub.l -B arranged near an optical axis receive equally divided light fluxes, paired light receiving elements 3.sub.m -A and 3.sub.m -B arranged remote from the optical axis receive differently divided light fluxes. Thus, even in an infocus condition the outputs from paired light receiving elements of the first and second element arrays differ from each other and thus the focus condition could not be detected accurately. Such a phenomenon is sometimes called an influence of image height.
In order to avoid such a drawback, it has been proposed to arrange a correction lens 4 between the imaging lens 1 and the means 3 for producing the laterally shifted images, i.e. the stripe mask as shown in FIG. 3. Such a correction lens 4 can guarantee the introduction of the uniform light fluxes onto the first and second light receiving elements arrays. However, in such a known apparatus, when the imaging lens is exchanged, the correction lens 4 can no longer compensate for the influence of image height optimally. Moreover, the correction lens 4 increases the number of parts and adjustments. Further, in some optical devices, it is difficult to find a space for arranging the correction lens.